This invention relates to detectors of remote movement and particularly a vibration detector in which a coherent light beam from, for example, a laser is directed at a remote target and light which is reflected from the target is mixed with reference light that is not so reflected to obtain a Doppler signal which represents the movement of the target. Such a detector of remote movement is particularly useful in the detection of movement of structural components of large bridges or other objects in locations which are not readily accessible for local measurement of their movement.
A serious problem which is encountered in the use of such a vibration detector is an undesirably large variation in the amplitude of the Doppler signal. This variation can be caused by local variations in reflectivity of the object or structure at which the beam of light from the laser is directed. The reflectivity may be inherent in the composition of the object or may be affected by atmospheric conditions. The variation can be so great as to cause the complete loss of the Doppler signal at times and if, as is often required, the Doppler signal is used to reproduce the waveform of vibration of the object the loss of signal seriously affects the accuracy of the measurement made by the detector.